Don't think of it as dropping Lacazette. Think of it as purposely starting Sanchez in a desperate attempt to improve their negotiating position. Their line had been that they are willing to keep him and risk losing him for free next year. Not playing him would have sent the opposite message and severely reduced any leverage that they have.
I'm very confident that Arsenal are resigned to selling him to us. They're just trying to squeeze as much money out of us as possible. They'll need it because that squad is not nearly capable of finishing top four. I expect Arsenal to do a lot of business over the next few days.